Enhancement of G protein-coupled signaling by DHA phospholipids

The effect of phospholipid acyl chain and cholesterol composition on G protein-coupled signaling was studied in native rod outer segment (ROS) disk and reconstituted membranes by measuring several steps in the visual transduction pathway. The cholesterol content of disk membranes was varied from 4 to 38 mol% cholesterol with methyl-β-cyclodextrin. The visual signal transduction system rhodopsin, G protein (G_t), and phosphodiesterase (PDE)] was reconstituted with membranes containing various levels of phospholipid acyl chain unsaturation, with and without cholesterol. ROS membranes from rats raised on n−3 fatty acid-deficient and-adequate diets were also studied. The ability of rhodopsin to form the active metarhodopsin II conformation and bind G_t was diminished by a reduction in the level of DHA (22∶6n−3) acyl chains or an increase in membrane cholesterol. DHA acyl chain containing phospholipids minimized the inhibitory effects of cholesterol on the rate of rhodopsin-G_t coupling. The activity of PDE, which is a measure of the integrated signal response, was reduced in membranes lacking or deficient in DHA acyl chains. PDE activity in membranes containing docosapentaenoic acid (DPA, 22∶5n−6) acyl chains, which replace DHA in n−3 fatty acid deficiency, was 50% lower than in DHA-containing membranes. Our results indicate that efficient and rapid propagation of G protein-coupled signaling is optimized by DHA phospholipid acyl chains.